Size 28, Resistance of 13.77 ohms per meter

sorry for asking this noob question, but what is the silicone's purpose? to hold wire in place? or make the surface even? or some unknown reason?

There are never stupid questions! Don't apologize.  Better ask then let something go wrong or break.

This silicone mat, is sold as being heat conductive to sit between heat sinks and heat sources (such as processors/electronic chips). So it helps to conduct and distribute the heat more evenly. It fully wraps around the nichrome wiring, contacting more surface area.

If there was no silicone the nichrome would be pressing directly against the glass and the alu bed, resulting in a very narrow contact patch, which brings along the second reasons. Electrical Insulation, I have the Kapton tape side down, facing the aluminium bed acting as an insulator. But also the silicone itself acts as an electrical insulator. Otherwise the nichrome wire would just short through the bed, not heat up and worse blow my power supply/mosfet switch.

The one long term safety mod I will do, is add a 7.5 amp fuse in series with the nichrome mat.

The third reason is as you state, to act as a shim/give some play and squish between the bed and the glass,.

I was thinking more like a two part silicone that you mix and then pour over the top with a rectangular frame to contain it. Should set flat, thin and with the wires nicely protected and insulated.

That should work, if you use my jig design, you would get peg holes, but they could be filled in later, or not bothered with.

Looking at the graph in Repetier, it seems to climb at around 20-25 degrees per minute.

Another question: how quickly does it heat up from cold to print ready?

Clever construction!

Seeing how neat you have it make me wonder about using a casting silicone over and around the wire to make your own heated mat. Would likely get a really neat result.

That did cross my mind, sandwiching it between 2 sheets of glass, with silicone from a chalk gun in between. It is hard to keep the nichrome holding its shape, it took my several attempts to get this all to work. Relying on kapton to hold it steady.

I did wander if using a heat gun on the nichrome would get it to hold its shape better when removed from the loom.

That would work, but I wanted to make this paper thin.

Kapton for the adhesive on the mat would probably work just fine.

Seeing how neat you have it make me wonder about using a casting silicone over and around the wire to make your own heated mat. Would likely get a really neat result.

The loom is great too, because it makes it easy to avoid certain sections.

Seriously looks great, I love the loom!

So by my maths I needed to make a 6 Ohm heating element, and make a new mosfet circuit to run it. I also wanted this to be as fully reversible as possible. So I found some wafer thin (0.5mm) silicone matting, and brought myself the other components. So the plan was/is to have it laid out Alu / Silicone / Nichrome / Silicone / Glass.

I needed to work out what nice paths I can make for the nichrome, that would get me the resistance I wanted. So pen and papered a couple of designs, until I got the length and runs needed. Then design a loom jig to form the nichrome into the correct shape.

Here is the jig:

I then placed some kaptop tape, to stick and hold the wires down:

Then do the other runs:

The lift it off, and stick the tape to the silicone:

Add some extra kapton, stick the edges down to the opposite side of the silicone, twist the ends up of the nichrome up, solder and heatshrink the cables on, and forget to take any more pictures. Until testing:

I use this in conjunction with the stock heater, giving me about 150 watts of heating in total, and also a more even heating area, which I hope should lead to less warping. I believe that as the heat element is at the centre of the bed, the circular drop off of heat to the outside exasperates the warping.

I have soldered a signal wire, from the gate pin of the Sanguinololu to signal my mosfet, both grounds of the power supplies are common.